The two men at the head of the coalition Government have traded blows over electoral reform as the referendum battle began in earnest.

With May 5 now confirmed as the date for a national poll on how MPs are elected to Parliament, David Cameron and Nick Clegg gave speeches within hours of one another arguing for opposing sides in the debate.

The Prime Minister warned that introducing the alternative vote (AV) system, under which voters rank candidates in order of preference, would be "a massive backward step for accountability and trust in our politics".

But his Liberal Democrat deputy insisted that getting rid of the current first-past-the-post system was the only way to tackle corruption at Westminster and prevent millions of voters being "ignored".

The coalition allies both stressed that the result of the ballot was not make-or-break for the Government. However, there were early signs of the potential tensions when Mr Cameron took Mr Clegg to task for supporting AV after previously branding it a "miserable little compromise".

Despite senior Tories and Lib Dems extolling the virtues of coalition over the past nine months, the premier also expressed concern that hung parliaments would become "commonplace".

It was "not necessarily a bad thing" for parties to come together in the national interest as happened last May, Mr Cameron told an audience in central London. But he warned: "When there are more hung parliaments there will be more haggling and horsetrading between politicians - both before and after elections."

If AV had been in use at the last election, Gordon Brown might still be Prime Minister, said Mr Cameron. He added: "Any system that keeps dead governments living on life support is a massive backward step for accountability and trust in our politics."

Speaking in Leeds earlier, Mr Clegg blamed the existing voting system for encouraging politicians to abuse expenses. "For years, politicians and parties have courted the votes of a few thousand people in marginal seats and ignored the rest," he said.

"It is because there are so many MPs with jobs for life that there are so many who can take their constituents for granted. And it is because there were so many MPs taking their constituents for granted that so many abused their expenses."